Radiation-curable binder compositions are used to create adhesion between two or more substrates. In addition to creating adhesion, binder compositions can contribute to the overall toughness of the bound substrates. Binder compositions which exhibit the property of high elongation at break can further contribute to the flexibility of the bound substrates; particularly when applied to substrates which alone are less flexible than the binder composition. The need for effective binder compositions in a wide variety of industries has been addressed in different ways depending upon the specific types of substrates which are bonded. For a wide variety of applications, binder compositions which exhibit strong adhesion, high durability and high elongation are most desired.
Binder compositions have a wide variety of uses in applications related to the binding of smooth surfaced substrates such as for example, glass substrates.
It is known that glass substrates can be weakened upon exposure to water. For example, moisture in air can cause weakening and the eventual breakage of glass. In particular, glass weakening may be accelerated under basic aqueous conditions. To prevent the destructive effects of moisture to glass substrates, a variety of polymeric coating compositions have been applied to a wide range of glass articles. The protective coating compositions can also act as an adhesive or binder composition to cement two separate glass substrates together. Radiation-curable compositions, whether used as coatings or adhesives are attractive because of fast cure speeds.
Coated and/or bonded glass articles include, but are not limited to, optical fibers (such as, for example, glass optical fibers), bottles, light bulbs, windows, safety glass (as used in automobiles and high-rise buildings), cemented compound optical lenses, and the like. Radiation-curable coatings which exhibit good binding characteristics are particularly important in optical fiber coating technology.
In addition to protecting and preserving the strength of the glass substrate, coatings can increase abrasion resistance of the glass substrate, protect the glass substrate from splintering if damaged, and in the case of coated glass optical fibers, improve glass substrate resistance to damage due to moisture and production methods.
Moisture, in addition to causing the weakening of glass substrates, can also cause polymeric coating compositions to break away or delaminate from the surface of the glass substrate. The delamination of a coating composition can result in direct exposure of the glass substrate to the harmful effects of moisture.
U.S. Pat. No. 5,000,541, issued to DiMarcello et al. teaches a method for hermetically sealing an optical fiber with carbon, which prevents water from contacting the optical fiber and thus prolongs the useful life of the optical fiber. Hermetic seals, however, severely limit production processes.
U.S. Pat. No. 4,849,462 issued to Bishop et al. teaches the incorporation of organofunctional silane into a coating composition to improve the adhesion between a coating composition for an optical fiber and the optical fiber, particularly in moist environments. The coating composition of Bishop does not teach the high elongation to break values of the present invention.
U.S. Pat. No. 5,502,145 issued to Szum discloses an optical fiber coating composition containing a hydrolyzable poly(siloxane) providing greater protection of the coated glass substrate from moisture damage. However, poly(siloxane) typically reduces adhesion of the composition.
Likewise, U.S. Pat. No. 5,214,734 issued to Inniss et al. teaches the incorporation of particulate silica in a polymeric coating composition to increase the fatigue resistance of an optical fiber or glass to moisture. However, the introduction of particulate matter into a coating composition as disclosed in the Inniss et al. patent can present problems such as scratching the pristine optical glass fiber, resulting in breakage at low tensile loads, a turbid coating composition, which has a tendency to gel, and other processing problems that are commonly encountered when working with particulate matter.
Further, many optical fiber coating compositions have other drawbacks that make them unsuitable for certain applications. For example, some of the compositions may be too expensive to use in the production of low cost optical fibers or glass objects.
In the field of optical glass lenses, extremely high precision is required in the bonding of two or more optical lens elements to form a single cemented compound lens. The quality of the cemented lens requires that the bond formed between the individual lens is durable, not easily degraded upon exposure to the environment and not subject to degradation over time.
In applications where the coating composition is bonded to large surfaces of glass substrate such as in the manufacture of safety glass the requirements for the binding composition may vary considerably. Safety glass, as used in automobile glass, skyscraper windows, or other such applications, can be formed as either a single coated glass substrate or multiple layers of glass sheets bonded together by an adhesive or binding composition. A major concern in the manufacture of safety glass is to use a coating composition which when bound to the surface of the glass substrate will exhibit strong adhesion, high durability and high elongation. Following a severe physical impact to safety glass the coated glass may break. It is desired that the coating composition or, in the case of multiple-layered bonded glass, the intermediate binder composition layer will remain bonded to the individual pieces of the broken glass and hold the broken pieces in close proximity to each other. It is the ability of the binding composition to undergo considerable elongation without breaking that keeps the broken glass from splintering and being scattered about in a hazardous manner.
U.S. Pat. No. 4,317,862 issued to Honda et al. teaches the use of photosensitive resins in binding compositions to provide a safety glass for use in vehicles. Prolonged effects of the environment and limited degrees of elongation detract from the effectiveness of binding compositions. Improved elongation of the binding composition is important to the improved safety of the bonded glass windows and articles produced.
An example of another major industrial area which requires the employment of binder compositions is the preparation of industrial sewing threads. Industrial sewing threads are utilized in numerous industrial applications and consumer products. For example, industrial sewing threads can be found in outdoor equipment such as tents, backpacks, sails, golf bags, and the like. Other products incorporating industrial sewing threads include shoes, carpets, rugs, automotive and aircraft upholstery, automotive safety bags, and the like. The durability, weatherability, flexibility and other characteristics of these products depend in large part upon the quality and nature of the industrial sewing thread. Industrial sewing threads are typically prepared from smaller fiber filaments or yarns. The individual filaments or yarns can be made of essentially any synthetic or natural material but are typically derived from, for example, various polymers such as nylons, polyesters, acetates, polyacrylonitriles, polyamides or aromatic polyamides such as KEVLAR from Du Pont de Nemours, E.I. & Co., or regenerated cellulose. The filaments and yarns can be wound, woven, or otherwise combined or integrated to form the larger industrial sewing thread fiber structure.
Individual fiber filaments or yarns can be bonded together with an adhesive, or binder composition, to form the thread. The binder composition creates adhesion between a significant number of individual fibrous structures and contributes to the overall toughness and flexibility of the sewing thread. Bonded sewing threads have several advantages including higher sewing rates, reduced time wasted due to thread break, increased thread abrasion resistance, and simpler twist of threads.
Heat-curable binder compositions are known which can be applied with use of volatile solvents and application of heat. However, this causes problems such as slow line speeds, energy costs for heating, and solvent emissions. Radiation-curable binder compositions, which do not employ volatile solvents and cure rapidly, may solve these and other problems. U.S. Pat. No. 5,409,740 discloses a dual-cure adhesive method which employs a resin composition which exhibits two curing mechanisms. The dual cure involves both a radiation cure (e.g., UV initiated radical polymerization of acrylate functional materials) and a moisture cure of isocyanate functional oligomers. This reference teaches that the moisture cure is indispensable. However, the moisture cure in particular makes this process unattractive because moisture cure requires time (e.g., 1-24 hr) to achieve good properties. Moreover, moisture cure can lead to inconsistent properties and undesirable high moisture contents.
U.S. Pat. No. 5,389,108 teaches that dyed materials can be exposed to ionizing radiation. However, radiation-curable materials are not disclosed. Other publications include U.S. Pat. No. 4,501,588, WO-A-94/25665 and EP-A-524,144, wherein radiation-curable compounds are used to achieve improved adhesion of coloring material to fibers. However, a binder for sewing thread is not provided.
In the articles, J. Appl. Polym. Sci. 1979, Vol. 23, No. 11, 3227-3242 and 3243-54, electron-beam curable compositions are disclosed. According to these articles, certain compounds can function in some cases to increase elongation. However, UV-curable compositions for binding industrial sewing thread are not disclosed.
It is an object of this invention to overcome disadvantages of prior art binder compositions and provide compositions which can be employed in the binding processes required for a wide variety of substrates to provide an outstanding balance of properties including, among others, elongation, toughness, secant modulus, and cure speed.